628 



LIGHT AND LIFE 



TABLE 4 



Cysteine-activated Photohvdrolysis of ATP 

 IN Spinach Chloroplasts 



A /xmoles Pi/hr/mg 

 chlorophyll 



Complete 



No light 



No PMS 



Complete + 5 X \0-^ M 



I 



HO- 



-O- 



-COO— butyl 



64 

 5 

 5 



7 



Reaction mixture. Each tube contained the following in ^imoles/2.7 ml: 1 ATP, 

 20 PEP, 20 MgCl2, 200 cysteine, 200 Tris buffer pH 7.7, 0.15 PMS, 5 jug dialyzed 

 pyruvic kinase, and spinach chloroplasts containing 0.25 mg chlorophyll. The incuba- 

 tion was for 20 minutes at 20°C in nitrogen. The reaction was stopped with 0.3 ml 

 of 50% TCA. Pi was measured in 0.3 ml aliquot. 



cysteine-activated ATP hydrolysis in chloroplasts is dependent on 

 light and PMS. The other characteristics of photohydrolysis also 

 appear to be alike in the algae and spinach jireparation. 



The effect of cysteine concentration on photoliydrolysis in chloro- 

 plasts is shown in Fig. 3. Cysteine does not increase the low dark 

 hydrolysis, but in the light the rate of dephosphorylation increases 

 proportionally with concentration up to 250 yu,moles cysteine in 3 ml. 

 Higher concentrations were difficult to test because of interference 



Cysteine Xffect on Photohydrolysis of ATP 

 ^ in Spinach Chloroplasts 



50 100 150 200 250 



^"M cy5teine/3 ml. 



Fig. 3. Cysteine-depcndent photohydrolysis of ATP in spinach chloroplasts. 



